1. Field of the Invention
The present invention relates to a filter circuit, and also relates to an image sensor, an image sensor module, and an image reading apparatus incorporating such a filter circuit.
2. Description of Related Art
In products, such as scanners and digital MFPs (multifunction peripherals) that incorporate a flat-bed-type image-reading apparatus, their construction often requires that signal lines be laid around over long distances, as is typically the case with the signal lines for connecting between a set circuit board and an image sensor module and the signal lines for connecting, within an image sensor module, between I/Os and an image sensor IC.
In particular, a contact-type image sensor module, which is so constructed as to have a plurality of image sensor ICs arranged end to end along the row in which photodetective elements are arranged, requires that signal lines be laid around over very long distances within the module. This makes it more likely that extraneous noise caught by the signal lines causes the logic portion of the image sensor ICs to operate abnormally and produces disturbance in the acquired image signal.
For this reason, conventional image reading apparatuses generally adopt configurations in which RC filters are inserted in signal lines for the purpose of eliminating extraneous noise.
As a conventional technology related to the present invention, there has been disclosed and proposed a filter circuit that is characterized by being provided with: a charge/discharge capacitor; an RC filter that is connected to the capacitor and that outputs an output voltage; a charge circuit that charges the capacitor when an input voltage is higher than the terminal voltage of the capacitor; and a discharge circuit that discharges the capacitor when the input voltage is lower than the terminal voltage of the capacitor (see JP-A-2002-190720).
As other conventional technologies related to the present invention, there have been disclosed and proposed a variety of digital filters that sample input signals a plurality of times and that choose one of the sampling results on a majority decision basis (see, e.g., JP-U-H5-059995, JP-A-H10-126228, JP-A-H11-195963, and JP-A-2002-185309).
To be sure, inserting RC filters in signal lines helps reduce the adverse effects of extraneous noise and thereby enhance the operating accuracy of image sensor ICs, and hence contributes to enhancement of the quality of the acquired image signal.
However, since the time constant (filter constant) of an RC filter depends on the arithmetic product of the resistance and capacitance of its constituent resistor and capacitor, when RC filters are integrated into an image sensor IC, the restrictions on the chip area naturally imposes restrictions on the filter constant. Thus, in a case where different filter constants need to be set in a wide range for different signal lines, RC filters need to be built with discrete components. This is one of the factors that have conventionally been hampering the efforts to reduce the device scale.
Incidentally, with the conventional technology disclosed in JP-A-2002-190720 mentioned above, it is possible to control the time constant and the input signal amplitude of a filter circuit without varying the resistance and capacitance used therein. This conventional technology, however, has the following disadvantage. According to the technology, an input signal is fed both to the supply voltage line of the charge circuit (current mirror circuit) and to the reference voltage line of the discharge circuit (current mirror circuit), and, according to which of the signal level of the input signal and the terminal voltage of the charge/discharge capacitor is higher, whether to operate the charge circuit and the discharge circuit is switched, and hence whether to charge or discharge the capacitor is switched. Thus, unless the driving capacity of the input signal is sufficiently raised, the charge circuit and the discharge circuit cannot be driven. This may produce an error in the charge/discharge current through the capacitor, causing the filter constant to deviate from the desired value.
On the other hand, with the conventional technologies disclosed in JP-U-H5-059995, JP-A-H10-126228, JP-A-H11-195963, and JP-A-2002-185309 mentioned above, it is possible to eliminate extraneous noise without the need for a resistor and a capacitor. Disadvantageously, however, these technologies are prone to malfunctioning in the presence of continuous pulse noise synchronous with the sampling clock of the input signal (i.e., noise containing pulses with pulse widths longer than the set-up and hold time of flip-flops).